The curing of coating systems carrying activated double bonds by radiation, such as e.g. UV light, IR radiation or electron beam radiation, is known and technically established. It is one of the fastest curing methods in coating technology. Adhesion is often a problem, however. In addition, the curing of coatings that can be cured by electromagnetic radiation is dependent on an adequate radiation dose. In badly lit or unlit areas, this leads to significantly poorer curing or to no crosslinking at all.
Binders based on polyisocyanates and polyols are extremely suitable for producing high-quality coatings. The desired paint properties, such as e.g. adhesion, elasticity, chemical resistance, weathering resistance or scratch resistance, can be adjusted within broad limits by varying the feed materials.
Combining these two mutually independent curing mechanisms in one binder system allows their positive properties to be united. Such systems, known as “dual cure” systems, are known. For example, US patent U.S. Pat. No. 4,342,793 describes the use of coating systems compounded from a radiation-curable reactive thinner, e.g. acrylic acid esters, a polyol and a polyisocyanate. The problem here is that in unlit areas the radiation-curable reactive thinner is left behind as a plasticiser and thus has a negative influence on the film properties or can even leave the film, which can lead to undesirable physiological effects.
Also known are “dual cure” binders whose radiation-curable components are chemically bonded to the polyisocyanate, so that the described effects can be avoided. For example, European patent application EP-A 0 928 800 teaches the use of NCO-functional urethane acrylates containing isocyanurate groups as a component of a “dual cure” coating system. In order to be able to apply these coating compounds easily, adequately low viscosities are needed, so various organic solvents are used.
Due the ecological and economic requirements for modern paint systems to use as little organic solvent as possible, if any, to lower the viscosity, there is a desire to use low-viscosity paint resins. Polyisocyanates having an allophanate structure have long been known for this purpose, as described inter alia in European patent EP-A 0 682 012.
Allophanates have long been known in coating compounds (see also GB-PS 994 890, EP-A-0 000 194) and are produced in industry by reacting a monohydric or polyhydric alcohol with excess aliphatic or cycloaliphatic diusocyanate. Unreacted diisocyanate is then removed by distillation in vacuo. Mixed, aliphatic/aromatic “heteroallophanates” are also known from EP 0 712 840.
EP-A 0867457 describes the production of allophanate-containing, radiation-curing binders based on polyurethane. However, these binders have no remaining free NCO groups, have viscosities of over 10,000 mpa·s at 25° C. (example 6-8) and have no activated double bonds, only unreactive allyl ether groups (structure R—CH2—CH═CH2). Reactive thinners (low-molecular esters of acrylic acid), which introduce the necessary UV reactivity, must therefore be added. These substances cannot be used for a dual cure process.
Isocyanate-containing dual cure binders having an allophanate structure and activated double bonds are described in the German laid-open specification DE-A 198 60 041. The process uses the conventional formulation with excess diusocyanate, the undesirable, unreacted monomeric diisocyanates having to be removed again in a subsequent, necessary film distillation. This procedure has the major disadvantage, however, that a high thermal loading during distillation at 135° C. can be expected, such that the activated double bond in particular tends to polymerise under these conditions. Lower temperatures are not enough to minimise sufficiently the proportion of diisocyanate monomers, which cannot be tolerated for health and safety reasons. The process is thus virtually unfeasible on an industrial scale.
EP-A 0 825 211 describes a process for synthesising allophanate structures by reacting diisocyanatooxadiazinetriones with phenols, saturated, aliphatic or cycloaliphatic or araliphatic monoalcohols or polyalcohols. Basic compounds, typically those with a pKa value of greater than 7.5, are used as a catalyst for the reaction. Stabilisation of double bonds is not described.
The underlying object of the present invention is to provide urethane acrylates and dual cure binders having activated double bond(s) which during their production or aftertreatment only require temperatures of below 100° C. In addition, particularly where volatile organic solvents (VOC=volatile organic compounds) are minimised or avoided, the desired binders should have sufficiently low viscosities of below 10,000 mPas at room temperature.